Sound proofing materials and structures have important applications in the transportation industry. For example, it is generally considered to be desirable to reduce the level of noise within a vehicle passenger compartment. Noises such as road noise, wind noise, engine noise, vibrations, and so forth, as well as noises emanating from within passenger compartments, can be attenuated through the use of various acoustically absorptive or reflective materials. Such materials can be included, for example, in headliners, trunk liners, hood liners, dash mats, interior panels, or carpeting and other decorative or functional vehicular facing materials to provide enhanced noise reduction in vehicle interiors.
Traditional materials used in the sound insulation industry, such as absorbers and reflectors, are usually active over a broad range of frequencies without providing frequency selective sound control. Active noise cancellation equipment allows for frequency selective sound attenuation, but it is typically most effective in confined spaces and requires an investment in, and operation of, electronic equipment to provide power and control.
Traditional sound-absorbing materials (for example, foams or fibrous materials) are generally relatively light in weight and porous and serve to dissipate the vibration energy of sound waves over their relatively large surface areas. Helmholtz resonators (comprising, for example, a layer of air sandwiched between two elastic substrates) can also be employed as sound absorbers. For both types of absorbers, however, relatively thick structures are generally required in order to obtain relatively good absorption characteristics at relatively low audible frequencies (for example, approximately 50 millimeters (mm) thickness for frequencies less than about 500 hertz (Hz)), and such thick structures can be problematic for use in confined vehicular spaces.
In contrast with sound absorbers, traditional sound barriers tend to be relatively heavy and air-tight because the sound transmission loss from a material is generally a function of its mass and stiffness. The so-called “mass law” (applicable to many traditional acoustic barrier materials in certain frequency ranges) dictates that as the weight per unit area of a material is doubled, the transmission loss through the material increases by 6 decibels (dB). The weight per unit area can be increased by using denser materials or by increasing the thickness of the barrier. Added weight, however, can be undesirable in many transportation applications.